Various resin compositions have so far been used as matrix resins for composite materials. Particularly in the field of thermosetting resins, epoxy resins have been used extensively in that these resins themselves are superior in mechanical properties, especially strength and elongation, and additionally good in adhesion to reinforcing materials and highly reinforced therewith.
However, there are restrictions of heat resistance, high-temperature mechanical properties, and water resistance on the use of epoxy resins. That is, epoxy resins have become unable to satisfy recent year requirements for high-performance composite materials with respect to heat resistance and other properties.
Thermosetting aromatic polyimide resins are known as heat resistant resins. Although excellent in heat resistance, these resins yield by-products in the curing reaction process, hence being inferior in processability. Bismaleimide compounds are also known as heat resistant thermosetting resins, but the products of curing these compounds alone are inferior in mechanical properties (particularly strength and elongation), hence these compounds being deficient in practicability. Such being the case, attempts have been made to addition-polymerize bismaleimides with diamines or the like. Products of curing by these addition process, although excellent in heat resistance, are brittle and since uncured compositions of these polymers have high melting points or glass transition temperatures, special high-boiling solvents are necessary when these compositions are used for impregnation, lamination, casting, adhesion, molding material applications, or the like, that is, there are problems in the processability of those compositions.
Recently, thermosetting resin compositions containing polyimide and alkenylphenol and/or alkenyl phenyl ether have been disclosed (Japanese Patent Application Kokai No. Sho. 52-994) as substitutes for the above mentioned resin compositions. While the products of curing these recently disclosed resin compositions are excellent in mechanical properties such as strength and elongation, they have problems such as low thermal-degradation resistance because of their hydroxy groups remain in the cured compositions. When allyl phenyl ether compounds which were disclosed in that patent are used, Claisen rearrangement reaction takes place by heating, so hydroxy groups are introduced in the cured compositions. In addition, these resins are deficient in reactivity and hence their processing requires long-time heating at high temperatures.
On the other hand, compositions composed of an allyl etherified substituted phenol novolak resin which is substantially free of hydroxy groups and bismaleimides are proposed (Japanese Patent Application Kokai No. Sho. 62-22812) as substitutes for the above resin compositions.
Although excellent in heat resistance and water resistance, the products of curing these resin compositions are somewhat inferior in mechanical properties such as strength and elongation and since the compatibility of the novolak resin with the bismaleimide is low, mixing of the two components requires long time at elevated temperatures. Moreover the resulting mixture, having high viscosity, possess a processability problem in that when these resin compositions are used as matrix resins for composite materials, the solventless impregnation with these compositions is difficult.